We have a few VM's on a Windows 2008 Server (Hyper-V) and are having a problem with routing between them.
The setup is that the Hyper-V server runs RRAS and maps IP's on its NIC to internal IP's (192.168.1.X) that the VMs use. The VMs use the hyper-V server as their gateway for outbound traffic. The reason for this setup is that our ISP assigns IP's by MAC address, so otherwise the VM's couldn't use the external IP's assigned to the server.
The issue is that the VM's cannot talk with each other using their external IP address. For example, if Server A is 126.96.36.199 (external IP)/192.168.1.1 (internal IP) and Server B is 188.8.131.52 (external IP)/192.168.1.2 (internal IP), you cannot ping 184.108.40.206 from 220.127.116.11. You CAN ping 192.168.1.1 from 192.168.1.2. We also have a Server C that is 18.104.22.168 (a different subnet), and that machine has no problem pinging Server A or Server B. So essentially unless the machines are on separate subnets, they can't talk with each other.
The reason we just don't use 192.168.1.X to communicate is that for this particular purpose we are setting up a monitoring server. This monitoring server will use a FQDN (like servera.myservers.net) to try to ping Server B. So we need to know if there is a DNS failure or something.
One weird thing is that if you do a tracert from Server A to Server C, you get a timeout for the first two attempts, and then a connection, but you don't see it going through a gateway.